Abstract

Diseases of plants caused by species of the genus Phytophthora represent some of the most devastating diseases of crops worldwide. All known species of Phytophthora are plant pathogens. Although superficially similar to filamentous fungi, the Phytophthora species belong to the class oomycetes, and are more closely related to brown algae. Phytophthora‐incited disease may be controlled by genetically encoded resistance in the host plants. In cases where no host plant resistance
to Phytophthora disease is available, application of control chemicals may be successful in controlling disease, although intensive chemical
control is potentially environmentally damaging. Much recent research into Phytophthora biology has utilised modern molecular biology techniques. DNA (deoxyribonucleic acid) sequencing of three Phytophthora genomes has revealed an extraordinary repertoire of genes encoding proteins secreted by Phytophthora for the purpose of overcoming and subverting host plant innate and active defence responses, indicating pathogenicity in
this genus to be a highly complex process.

Key Concepts

Species of Phytophthora cause some of the most destructive plant diseases known.

Phytophthora species resemble fungi, but are phylogenetically distinct.

Some Phytophthora species have coevolved with their hosts, leading to a gene‐for‐gene system of matching plant resistance and Phytophthora avirulence gene products.

Some, but not all, Phytophthora diseases may be controlled by resistance in the host plant, or through the application of control chemicals.

Phytophthora species secrete a complex array of proteins, including effectors, during invasion of plant tissues, some of which act to
suppress plant defence responses.

Some effectors may be recognised by host plants, leading to resistance.

Other effectors may be required for pathogen growth and development during infection.

Major lifecycle stages of Phytophthora, with Ph. infestans used as example. Asexually formed sporangia (a) are multinucleate and their cytoplasm may cleave to form motile zoospores
(b). Encystment occurs next (c), followed by cyst germination and formation of the appressorium (d) for infection of the host
plant. Following penetration into the host plant, intercellular hyphae are formed with occasional formation of haustoria ((e);
trypan blue staining). After plant tissue is fully colonised, hyphae emerge from host surfaces to form sporangiophores bearing
more sporangia ((f); trypan blue staining). The formation of sporangia is most conspicuous when it occurs as a mass of white
aerial hyphae and sporangiophores (g) on aerial parts of the plant. Oospores (h) are sexually derived spores formed when opposite
mating types (for heterothallic species) meet in the same diseased plant. Homothallic species form oospores continually without
the requirement for presence of another mating type. Zo, zoospore; cy, cyst; gc, germinated cyst; ap, appressorium; ih, intercellular
hyphae; ha, haustorium; oo, oogonium and at, antheridium. Scale bars represent 50 μm (a), 20 μm (b, c, f), 10 μm (d), 30 μm
(e) and 40 μm (h).